There are many circumstances where large batteries may be found. In particular, large batteries or battery installations can be found both in stationary applications or for traction purposes, such as in standby DC power installations from which invertors and the like may be run; or in traction devices such as fork lift trucks and pallet trucks, golf carts, and electrically powered passenger and cargo vehicles. Large batteries may be configured from a plurality of cells or modules, which may be connected in series so as to develop a substantial terminal voltage across the battery, and there may often be a parallel connection of other modules or a series configuration of parallel connected cells or modules.
The present invention is particularly directed to large batteries which are configured in a series configuration of individual cells or modules. It is the purpose of the present invention to provide means for monitoring the condition and performance of the thus configured batteries, and particularly when such batteries may be utilized in electric vehicles. Typical battery configurations of experimental electric vehicles currently found in the public include the General Motors IMPACT, which utilizes a battery having a battery voltage of 320 V, and on the other hand a 72 V battery which is found in the PANDA ELETTRA manufactured by Fiat.
The present inventor is also the inventor in U.S. patent applications Ser. Nos. 07/253,703 and 07/676,523 filed Oct. 6, 1988 and May 2, 1991, respectively, each in respect of battery chargers which are capable of providing very high rates of charge to batteries. The charged batteries may, in some instances, have very high capacities. Each of those applications is assigned to the common assignee herewith. A further co-pending application in the name of the present invention, and with a common assignee, is directed towards charging stations at which electric vehicles may have their batteries recharged in a very short period of time. Of course, it is important that the batteries be monitored not only while they are being charged, but during discharge as they are in operation, because of the high current flow to which the battery may be subjected. This is particularly evident when it is noted that a standard of only a few years prior to the filing of the present application was generally that batteries would be charged at a rate of about 0.1 C--in other words, at a current which required 10 hours to recharge the battery; whereas it is now feasible to charge very high capacity batteries at rates of 3 C to 10 C--in other words, in as little as from 6 minutes to 20 or 30 minutes. Clearly the risk of overcharge of batteries being charged at such rates is higher, and thus the risk of damage to the cells or modules of the battery is higher. Likewise, batteries that are used in traction circumstances, in particular, may be required to be discharged at very high rates of current, possible over sustained periods of time.
In a series connected battery, the same current passes through all of the cells in the battery, or all of the battery modules in a large, high voltage battery, both while the battery is being charged and when it is being discharged. There are electrical means available by which weak cells or modules of the battery may be bypassed, but those means are cumbersome and costly, and generally result in a lower operating terminal voltage of the battery; and they are therefore impractical. Thus, the present invention addresses battery configurations where all of the cells or modules of the series configured battery are active at all times.
Several significant advantages are provided by the present invention, which are to the benefit of series connected batteries. They include the ability to provide for adjustment of the charging current or the discharge current in keeping with the ability of the weakest battery module or cell to pass the adjusted current, thereby preventing abuse and prospective irreversible damage to the module or cell. Likewise, the present invention provides means by which it is capable to diagnose and identify a weakening battery module or cell, long before failure of the module or cell occurs. By such early detection and identification of a weak battery module or cell, the defective battery module may be replaced at the earliest opportunity, thereby restoring the battery to its full useful and operative capacity.
These features of the present invention are achieved by monitoring the cell electrochemical potential--in other words, the resistance free voltage of the cell or module--as well as in some circumstances monitoring the cell or module resistive drop--i.e., the voltage drop across the module or cell which may be attributed to and is a consequence of the ohmic resistance of that module or cell. Such monitoring is not only possible, but in keeping with provisions of the present invention, it is conducted during all conditions of the battery operation; in other words, during both charge and discharge of the battery. It will be noted in the following discussion that there may be more circumstances where the battery operation and condition is monitored during charge rather than being monitored during discharge, arising at least in part because the battery is usually in a fixed place while it is being charged. Moreover, the charging operation may be under the control of a charger which is fixed or stationary, and therefore capable of monitoring a plurality of batteries as they may be individually connected to the charger. This charge monitoring is, however, of significant value and is fully contemplated by the present invention.
The present invention recognizes that the resistance free voltage of a battery or of a cell or module within the battery is the key characteristic by which a determination may be made as to the state of charge of the module or cell, and indeed as to its overall condition and its ability to undergo electrochemical activity--charging or discharging. Moreover, as noted in the co-pending applications directed to battery charging, the high limit and relative value of the internal resistance free voltage of a battery module or cell is utilized to control the charging process for that battery.
It is stressed that the present invention recognizes that, particularly in installations such as an electric vehicle, the low limit of the resistance free voltage of a battery cell or module should be monitored during discharge so as to permit control of the discharge current in order that it may be gradually reduced while a warning is given to the vehicle operator, and eventually so as to terminate the discharge operation by inhibiting the traction controller and other loads on the battery, at a predetermined threshold. The analogy is that the first condition noted above is similar to that of a "low fuel" indicator in an ordinary petroleum fueled vehicle whereas the second condition is analogous to a fuel tank "empty" condition. Abuse and damage to the battery is thereby precluded.
Upon analysis, it is obvious that close monitoring of the charge and discharge conditions of a battery are superior to simply monitoring the battery voltage, since it permits the battery to more closely approach its fully discharged condition without running into the prospective danger of overdischarge. This is especially so if, in keeping with the present invention, the condition or state of the weakest cell or module of the battery is that which is used to determine the instant at which either a warning of impending module or cell failure should be given, or battery discharge operation is terminated.
Moreover, the present invention also recognizes that resistive drop across the battery cell or module while current is passing through it on either charge or discharge, provides a direct indication of the amount of irreversible resistive heat which is evolved in that cell or module as a consequence of the current flow. Upon analysis, it is clear that an increasing resistive voltage drop across the cell or module is an indication of an increasing cell or module resistance. That increased cell or module resistance may be caused such as by deterioration of the electrodes in the cell or module, or by poor internal or external contact of any current carrying component, or even as a consequence of loss of electrolyte in the cell or module.
The thesis that parallel connection of batteries is not recommended for fast charging of the batteries, is presented herein. Current sharing among parallel chains within a battery is an unstable equilibrium; and without some further additional electrical means to force equal current sharing within a parallel connection--which additional means is cumbersome and expensive, causing additional energy losses, and unnecessary--the parallel connection of a battery while being charged is discouraged. The reasons are explained below:
It must be kept in mind that the cell voltage of most known practical battery systems exhibits a negative temperature coefficient during charging. Moreover, even with the most stringent manufacturing controls, there are minor differences among otherwise equal cells in a battery, or among equal battery modules in a large scale battery. Any battery cell which becomes marginally warmer in a parallel connection, under charge conditions, will exhibit a lower electrochemical potential because of the negative temperature coefficient, therefore that cell presents a lower electrochemical resistance to the charge current. Thus, the non-conforming cell presents a preferential path to the charging current; and as a consequence of that circumstance, even further increased current will flow to the cell and it will become yet again warmer. That encourages still higher charge current than through any other cell parallel to it, and the effect continues to spiral. Thus, one battery cell or module will become hot and overcharged, while the remaining battery cells or modules in parallel to it remain cold and undercharged. The only possible manner by which that problem can be overcome is to provide good thermal coupling, with a time constant that is substantially shorter than the charge time--for example, among the electrodes in a multiple plate cell. In that case, then the heat is transferred fast between the non-conforming unit and its surrounding and parallel neighbours, so that all of the parallel cell elements remain at substantially the same temperature. Such conditions are clearly not practical in the context of large scale, high voltage batteries.
On the other hand, when a battery is being discharged, it is recognized that a stable equilibrium may be presented in current sharing circumstances among parallel cells or modules. Those features are, however, outside the scope of the present invention.
What the present invention does provide is a monitoring circuit for a battery, whether the battery is being charged or discharged, where the battery comprises a series of modules that are connected in series with terminals placed between each adjacent pair of modules and at each end of the battery, so that the voltage across each module may be measured. In one aspect of the present invention, means are provided for periodically interrupting the flow of current to or from the battery for an interruption interval of fewer than about 2 to 5 ms, or up to 10 ms for installations such as large traction batteries; and means are also provided for selectively sampling any one of the modules during an interruption of current flow, so as to make a determination of the resistance free voltage of that module. A selected plurality, if not all of the modules, could under some circumstances be individually sampled during any one interruption of the current flow. In any event, means are provided for comparing the resistance free voltage of any module that is sampled against at least two reference voltages, so as to determine if the resistance free voltage is above a first predetermined level or below a second predetermined level. The present invention further provides means for causing an alarm indication in the event that the resistance free voltage is found to be either above the first predetermined level or below the second predetermined level.
In a further embodiment of the present invention, further means are provided for sampling the voltage across any one of the modules at a time while current is flowing through the module. That sampling may occur during charge or discharge. Sample and hold means are provided for determining and temporarily holding the values of a first voltage across the module being sampled when current is flowing through the module, and also to sample and hold the value of a second resistance free voltage across that same module during an interruption interval of current through the module. Means are provided for determining the relative magnitudes of the first and second voltages to each other, and for determining the higher of the first and second voltages during charging and discharging operations of the battery. This is accomplished by determining the absolute value of the differences between those two voltages during charge and discharge conditions; noting that under charge conditions the first voltage should have an absolute value higher than the second voltage, and that under discharge conditions the circumstances would be reversed. Further alarm means are provided to give an alarm indication in the event that either of the first or second voltages is above or below predetermined values, or in the event that the relative magnitudes of the first and second voltages to each other are beyond predetermined limits.